Hitherto, the application of not only ceramics or glasses having a low thermal expansion coefficient but also cheap stainless steel having superior heat resistance to an insulating substrate is considered.
For example, PTL 1 and 2 disclose insulating materials in which a smooth surface of a stainless steel sheet is coated with an alumina, silicon oxide, or silicon nitride film. As these materials, commonly-used ferritic stainless steel SUS430 (17Cr steel) is used.
Further, PTL 3 discloses a material with a stainless steel surface having superior film formability in which both surface roughness parameters Rz and Rsk are specified. As this material, SUS430J1L (18Cr-0.4Cu-0.4Nb) to which Nb and Cu have been added and commonly-used austenitic stainless steel SUS304 (18Cr-8Ni), are used.
Recently, photovoltaic power generation has been developed as a major energy alternative to fossil fuels, and the development of techniques for a solar cell has been accelerated. Among these techniques, it is expected that a CIS (chalcopyrite) solar cell will be widely used in the future as a solar cell in which low cost and high efficiency are simultaneously realized. In the CIS solar cell, an electrode layer formed of Mo is formed on a substrate, and a chalcopyrite compound layer as a light-absorbing layer is formed on the electrode layer. The chalcopyrite compound is a quinary alloy represented by Cu(InGa)(SeS)2.
In the related art, glass which is an insulator having a low thermal expansion coefficient has been widely used for a substrate for a solar cell. However, since glass is brittle and heavy, it is difficult to mass-produce a substrate for a solar cell in which a light-absorbing layer is formed on a glass surface. Therefore, recently, the development of a substrate for a solar cell using stainless steel which has superior heat resistance and a good balance between strength and ductility, has progressed in order to realize weight reduction and mass production.
For example, PTL 4 discloses a method of producing a substrate for a solar cell, the method including: forming an insulating film formed of alumina on stainless steel foil having a thickness of 0.2 mm or less; forming an electrode formed of Mo on the insulating film; and forming a Cu(In1-xGax)Se2 film as a light-absorbing layer on the electrode. As the stainless steel foil, SUS430, SUS444 (18Cr-2Mo), or SUS447J1 (30Cr-2Mo) is used.
In addition, PTL 5 and 6 discloses electrode substrates for a CIS solar cell in which a Mo film is formed on a Cu coating layer of a Cu-coated steel sheet, and a Cu(InGa)(SeS)2 compound layer is formed on the Mo film. PTL 5 and 6 discloses that ferritic stainless steel is used as a base material of the Cu-coated steel sheet, the ferritic stainless steel containing C: 0.0001% to 0.15%, Si: 0.001% to 1.2%, Mn: 0.001% to 1.2%, P: 0.001% to 0.04%, S: 0.0005% to 0.03%, Ni: 0% to 0.6%, Cr: 11.5% to 32.0%, Mo: 0% to 2.5%, Cu: 0% to 1.0%, Nb: 0% to 1.0%, Ti: 0% to 1.0%, Al: 0% to 0.2%, N: 0% to 0.025%, B: 0% to 0.01%, V: 0% to 0.5%, W: 0% to 0.3%, a total amount of Ca, Mg, Y, REM (rare earth elements): 0% to 0.1%, and a remainder including Fe and unavoidable impurities. However, the ferritic stainless steel used in Examples is limited to SUS430.
PTL 7 discloses stainless steel on which an insulating film having superior heat resistance is formed, and a method of producing the same. PTL 7 discloses that the stainless steel as a substrate contains C: 0.0001% to 0.15%, Si: 0.001% to 1.2%, Mn: 0.001% to 2.0%, P: 0.001% to 0.05%, S: 0.0005% to 0.03%, Ni: 0% to 2.0%, Cu: 0% to 1.0%, Cr: 11.0% to 32.0%, Mo: 0% to 3.0%, Al: 1.0% to 6.0%, Nb: 0% to 1.0%, Ti: 0% to 1.0%, N: 0% to 0.025%, B: 0% to 0.01%, V: 0% to 0.5%, W: 0% to 0.3%, a total amount of Ca, Mg, Y, REM (rare earth elements): 0% to 0.1%, and a remainder including Fe and unavoidable impurities, in which a mixed layer of NiO and NiFe2O4 having a thickness of 1.0 μm or more is formed on a surface of the substrate with an Al oxide layer interposed therebetween. Steel used in Examples is Al-containing ferritic stainless steel containing Si: less than 0.4%. In addition, PTL 7 describes that the Si content in the steel may be controlled to be 0.5% or less. In addition, the mixed layer of NiO and NiFe2O4 and the Al oxide layer are formed by forming a Ni plating layer through electroplating, and then forming an Al oxide layer at an interface between the steel and the Ni plating layer and modifying the Ni plating layer into an oxide layer through a heat treatment in air.
On the other hand, PTL 8 and 9 disclose methods of producing stainless steel in which insulating properties are imparted to a stainless steel surface without using a method of coating a coating material. PTL 8 discloses a method of heating a ferritic stainless steel sheet containing 2% or more of Al to 850° C. or higher to form an aluminum oxide layer thereon. However, in Examples, the time of a heat treatment on steel obtained by adding Al to SUS430 containing impurities such as C or N is limited to 60 minutes. In addition, PTL 9 discloses stainless steel whose entire surface is coated with α-Al2O3 containing equiaxed crystals or columnar crystals by performing an oxidation treatment thereon at 1000° C. for one hour or longer. However, the stainless steel used in Examples is limited to 20Cr-5Al.